There are a variety of uses for fine copper (Cu) powder, for example in Plasma Display Panels, Field Emission Displays, automobile lights and the like. For example, Cu powder is formulated into an electrically conductive metal paste material, which may be conductive on compression or alternatively on sintering, where said paste is employed in multilayer passive devices, for example, a multilayer ceramic chip capacitor. Generally, micron-sized particles are useful for conductive pastes, such as described for example in U.S. Pat. No. 4,735,676, U.S. Pat. No. 4,997,674, and U.S. Pat. No. 5,011,546. The current generation of multilayer integrated circuit devices preferably utilize sub-micron copper powder, e.g., with a particle size ranging from 0.8 microns to about 0.1 microns, for example to produce the conductive material for inner electrodes on integrated circuits.
Many different methods have been proposed in the synthesis of a copper powder used in the conductive paste as described above, but they can generally be classified as either a gas phase method and a liquid phase method. Conventional methods for manufacturing metal powders have various problems such as a low yield due to wide particle size distribution, large particle size, low sphericity, and difficulty in controlling a degree of oxidation.
The gas phase method, also known as the gas atomization method, involves forcing high-pressure inert gas and molten copper through a nozzle with sufficient velocity to “atomize” the liquid metal, which on cooling yields a metal powder. Although this method is suitable for mass production, it is difficult to manufacture a nano-scaled powder (e.g., particles with a diameter equal to or less than 0.15 microns) with a commercially acceptable yield by this method. To obtain commercially acceptable product, oversized particles must be separated from the particles having a diameter in the preferred range. Such processes are difficult because powders are often irregularly shaped and separation is therefore difficult.
There is also a gas phase thermal decomposition method, where a copper-containing salt that has a weak binding force between metal and anion is thermally decomposed using a gas reducing agent and milled to obtain a metal powder. This method provides a fine metal powder. However, the metal powder may be burned during a heat treatment the burned powder is required to be milled and classified. Therefore, this method has a lower yield than a liquid phase reduction method.
In a gas phase evaporation method, an evaporation material is evaporated by heating in an inert gas or an active gas such as CH4 and NH4, and the evaporated gas is reduced with hydrogen and condensed to obtain a fine metal powder. This method is useful in preparing a metal powder having its particle size of 5 nm to several microns. However, productivity is very low and thus the metal powder is very expensive.
A liquid phase reduction method is a well-known chemical method for manufacturing a metal powder. This liquid phase reduction method can more easily control the shape of the powder. Typically, a metal powder is prepared by a procedure comprising 1) forming a soluble first intermediate, 2) producing an insoluble intermediate product, and 3) adding a reducing agent. A conventional liquid phase reduction method for preparing a copper powder first has copper oxide (copper hydroxide which is then dehydrated to form CuO) precipitated by adding sodium hydroxide (NaOH) to an aqueous copper sulfate solution, and the slurry is then filtered to separate particles from liquid. In a second step, a stable Cu2O solution is obtained by reacting the CuO with glucose or other monosaccharide having 6 carbons and an aldehyde group. When the color of the resulting solution changes to a dark red due to the production of Cu2O, glycine and arabic gum are added to control the size and surface shape of the final copper powder. Then, a reducing agent, typically formalin or hydrazine, is added to reduce Cu2O to obtain a copper powder. The particle size of the copper powder varies depending on the conditions existing when each of the many reagents and additives are added, and thus it is difficult to control the particle size.
U.S. Pat. No. 6,875,252 teaches a method of producing a copper powder that the application states has an average particle diameter in the range of from not less than 0.1 micron to less than 1.5 microns, “preferably” between 0.3 to 1.2 microns. The examples, however, showed the minimum size obtained was in fact 0.8 microns using the process described in the patent, and 0.6 using prior art processes. The copper powder is produced by wet reduction of cuprous oxide into metallic copper powder in the presence of ammonia or an ammonium salt. The size of the copper powder is related to the size of the copper hydroxide formed in the first step and also to the size of the copper(I) oxide formed in the secondary reduction. In particular, an aqueous solution of a copper salt and an alkali are reacted to precipitate copper(II) hydroxide. A first reduction step is then conducted in the suspension to reduce the copper(II) hydroxide obtained to cuprous oxide. This first reduction step is performed by adding glucose (a reducing agent) to the obtained copper(II) hydroxide suspension in order to reduce the copper(II) hydroxide to cuprous oxide in the ordinary manner. This first reduction step is preferably carried out under an inert gas atmosphere and increasing temperature (50-90° C.). By blowing an oxygen-containing gas into the suspension of cuprous oxide produced by the primary reduction, the particle diameter increases but the particle size distribution width is narrowed. Then, a second-reduction step is conducted in the suspension to reduce the cuprous oxide obtained to metallic copper, wherein the second reduction step is started by adding about 0.01-0.1 moles ammonia per mole of copper and 1.1 times the chemical equivalent of hydrous hydrazine required for reducing the cuprous oxide to metallic copper. High density smooth surfaced metallic particles produced from this process enable the electrodes to form into solid sintered bodies with few pores by sintering at a low temperature.
U.S. Pat. No. 6,673,134 teaches a method of producing a flaky copper powder having an average major axis diameter of 4 to 10 microns and a flakiness of 2 to 20, where said flaky powder is produced by introducing a copper slurry having fine granular copper particles having an average particle size of 3 to 5 microns dispersed in water into a bead mill containing zirconia beads having a diameter of 0.3 to 1.0 mm and milling, thereby flattening said copper powder. This patent describes the known art, stating “wet synthesis provides copper powder having an average particle size regulated between about 0.2 to 4.mu·m with a narrow particle size distribution but involves high cost and has an economical problem.” The patent teaches that after milling “the pulverized copper powder is classified by a classifier (and) the desired fine copper powder is discharged and collected by a cyclone or a bug filter (while) coarse copper powder is fed back to the milling chamber and pulverized again.” Such a process does not provide the desired narrow particle size nor the usually desired solid (not flaky) particles.
U.S. published Application 20040221685 (now abandoned) describes a method for manufacturing a copper powder by a wet reduction process, comprising adding appropriate amounts of sodium hydroxide and hydrazine to an aqueous copper chloride solution to finally obtain a copper powder having a particle size of 0.1 microns. A first method for manufacturing the copper powder includes the steps of: (1) adding sodium hydroxide to an aqueous copper chloride solution to give an aqueous solution containing copper oxide and copper hydroxide; and (2) reducing the copper oxide and the copper hydroxide to copper powder by adding hydrazine to the aqueous solution, wherein the composition is kept within a temperature of 40° C. to 80° C. In an alternative method, in an intermediate step, a stable Cu2O solution is obtained by reacting the obtained CuO with an aldohexose (a monosaccharide having 6 carbons and an aldehyde group) such as glucose. An amino acid, e.g., glycine, and arabic gum are added to the Cu2O solution to control the size and surface shape of the final copper powder. This patent also describes forming a complex of hydrazine (an amine) and soluble copper salts, and then precipitating copper powder by admixing therein an alkali.
U.S. Pat. No. 5,094,686 teaches a method of producing a powder which comprises thermally decomposing anhydrous copper formate in a solid phase in a non-oxidizing atmosphere at a temperature in the range of from 150° to 300° C., thereby yielding a copper fine powder having a primary particle diameter of from 0.2 to 1 microns, a specific surface area of from 5 to 0.5 m2/g and small agglomerating properties.
Preparation of Very Finely Divided Copper by the Thermal Decomposition of Copper Formate Monoethanolamine Complexes, Kimchenko, Y. I., et al, Poroshkovaya Metallurgiya, No. 5(245), pg. 14-19 (May 1983) describes and compares the processes of forming copper powder by the thermal decomposition of copper formate versus the thermal decomposition of a copper-monoethanolamine formate complex. Monoethanolamine (MEA) is a known alternative to ammonia to form aqueous soluble complexes of copper. To get high concentrations of the dissolved complex in the solution, there should be a supply of anions to form a stable copper-MEA-anion complex, and commercially the anion is carbonate, chloride, nitrate, borate, citrate, sulfate, acetate, or the like. Low molecular weight organic acids such as formic acid and oxalic acid are a known reducing agent. In this work, the composition did not comprise much water, as copper formate dihydrate was dissolved in straight MEA to form the starting mixture. Formation of metallic copper by thermal decomposition of copper formate dihydrate (or alternatively from copper oxalate) is known. When decomposing copper formate, there are two isotherms shown in a differential thermal analysis. The first, hitting a maximum at about 380° K (107° C.) corresponds to the dehydration of the dihydrates, while the second, hitting a maximum at about 453° K (180° C.) corresponds to the decomposition of formate and the formation of metallic copper. When a copper-monoethanolamine-formate solution is used, the differential thermal analysis shows five endotherm effects. The first isotherm at 384° K (111° C.) relates to dehydration, the second isotherm at 405° K (132° C.) relates to detachment and removal of 1 mole of MEA, and the third isotherm at 419° K (146° C.) relates to decomposition of the complex and the formation of metallic copper (formed at temperatures as low as 139° C.). The remaining isotherms relate to boiling off/condensing the remaining organics. While this method is useful, the use of copper formate as a precursor is expensive. Further, the paper noted the resultant copper powder had, as a result of uncompensated surface forces, crystal lattices in a state of dis-equilibrium, having macro- and micro-stresses therein.
There is a need for cost-effective method of preparing stable copper powder that does not require one or more low molecular weight organic acids, e.g., formate ions and/or oxalate ions, or expensive and unstable reducing agents such as hydrazine, for each copper ion.